For decades, scientists have debated the possibility that a vital ocean current system could plunge Europe into an abrupt cooldown.
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In a 1998 Atlantic cover story, William H. Calvin offered perhaps the best oceanography lesson to appear in a major national magazine. It was also a call for concern: He drew on the research of the legendary Columbia University climate scientist Wallace Broecker to explain the relationship between ocean currents and the climate, and warn about a rather counterintuitive tipping point that our age of global warming could cause. By warming the planet, humanity might kick off a disastrous oceanographic flip-flop.
A part of the great underwater conveyor belt called the Atlantic Meridional Overturning Circulation, better known as AMOC, could shut down, he wrote. Enormous inputs of freshwater from melting northern ice or warming-induced rainfall in the high latitudes could dilute the salty Atlantic Ocean and change the temperature balance, throwing off the pace of the North Atlantic Current, which makes up a section of AMOC’s global journey. Instead of warming, the Northern Hemisphere then would sharply cool, plunging Western Europe in particular into a prolonged and lethal deep freeze. The North Atlantic Current “keeps northern Europe about nine to eighteen degrees warmer in the winter than comparable latitudes elsewhere—except when it fails,” Calvin wrote.
This has precedent: Thanks to sediment cores, scientists know that this scenario happened some 12,800 years ago. Back then, the world had been heating up, and half the ice sheets that covered Europe and Canada had melted. Then the temperatures suddenly dropped. “The return to ice-age temperatures lasted 1,300 years,” Calvin wrote. Should it happen again, this “abrupt cooling” could cause mass chaos and death, ushering in a dark, cold age that could last more than 1,000 years:
To the long list of predicted consequences of global warming—stronger storms, methane release, habitat changes, ice-sheet melting, rising seas, stronger El Niños, killer heat waves—we must now add an abrupt, catastrophic cooling. Whereas the familiar consequences of global warming will force expensive but gradual adjustments, the abrupt cooling promoted by man-made warming looks like a particularly efficient means of committing mass suicide.
That abrupt cooling could result from dangerous warming may sound more like the muddled arguments of climate deniers today than it does established climate science. Yet it is a legitimate potential outcome of the melting of the Greenland ice sheet, which is currently dumping some 30 metric tons of meltwater into the North Atlantic per hour. This input could disrupt the balance of heat and salt in the northern ocean that keeps AMOC churning.
We now know that AMOC has weakened somewhat over the past century. But a dramatic slowing or halting of this major underwater conveyor belt, which circulates heat and salt around the Atlantic Ocean, could trigger dramatic cooling in Europe, drought in the tropics, rapid sea level rise off the southeastern U.S. coast, and changes to the monsoon in India and East Asia—not to mention impacts to marine ecosystems that could threaten food supplies. The debate over whether or when this could happen persists in the climate community; just last year, a paper in the journal Nature suggested that a collapse could take place sometime this century.
Yet other researchers are more hesitant to give too much air time to this particular doomsday scenario. Most models still show AMOC slowing down, but not its total collapse. When I contacted Calvin, a neurophysiologist and the president of the CO2 Foundation, about his cover story’s current relevance, he noted that AMOC’s decline “was, before 1998, a strong possibility that motivated climate scientists and funding agencies; now, after another quarter-century of effort, the slow decline is well-established.” Yet there remains considerable uncertainty as to “where and how fast it can happen, as no one understands the cooling dynamics.” So is there a deep freeze of Europe on the horizon? The answer, for Arnold Gordon, an oceanographer at the Lamont-Doherty Earth Observatory of Columbia University, is a resolute no.
“I don’t think that the climate warming now is going to shut down AMOC,” he told me over the phone last week. “It may slow up in the coming decades. But I think it’s going to go right back up again.” Gordon’s optimism stems from his belief that even the much-improved modern ocean models don’t properly capture particular dynamics of salt in the Atlantic Ocean that stand to benefit AMOC. The Atlantic is significantly saltier than the Pacific, and salinity differences help drive the AMOC conveyer belt.
The “abrupt cooling” scenario would result from that salinity dropping, diluted by meltwater from disappearing northern ice. But, Gordon said, that projection—and the models that predict it—fail to take into account the Agulhas Leakage, an inflow of warm, salty water from the Indian Ocean into the South Atlantic. That ocean dynamic can take the form of giant eddies, which he said models are notoriously bad at capturing. But once the salt from the Agulhas Leakage reaches the northern Atlantic, he predicted that AMOC would start back up again. “They’re not simulating an important part of the salt that goes into the Atlantic,” he explained. “We need models with higher resolution to really look at this.”
Plus, more surface heating due to global warming will evaporate seawater at faster rates, leaving salt behind and increasing the remaining water’s salinity. “More evaporation and more leakage,” Gordon said. “Those two things together will counteract the freshwater inputs in the Northern hemisphere.” Even if Greenland melts faster than expected, as it now seems to be doing, he predicted that the current may slow and the salty water may “take a little bit of time” to restore its flow, but it will right itself within decades. Europe isn’t about to become an ice cube for 1,300 years again, in his view. Many researchers seem to be somewhere in the middle: concerned, but not panicked over the fate of AMOC. Even Broecker, the scientist who first theorized that a reorganization of ocean circulation triggered the dramatic changes in the North Atlantic during the last ice age, didn’t think a modern version would be nearly as dramatic.
Yet the gulf between Gordon’s optimistic view and the gloom of recent literature warning that an AMOC shutdown is possible or imminent is more evidence of just how little we know about the complex dynamics that govern global ocean currents. Ocean-circulation models have improved dramatically since Calvin published his Atlantic story in 1998, but as scientists are known to say: More research is needed. What’s clear is that the collapse of AMOC is not a point we want to get anywhere near reaching; the consequences of not understanding global ocean dynamics leave us gambling with our own future.
“Medieval cathedral builders learned from their design mistakes over the centuries, and their undertakings were a far larger drain on the economic resources and people power of their day than anything yet discussed for stabilizing the climate in the twenty-first century,” Calvin wrote 26 years ago. “We may not have centuries for acquiring wisdom, and it would be wise to compress our learning into the years immediately ahead.”
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